As electronic equipment becomes portable and wireless, compact and lightweight lithium rechargeable batteries having high energy density are being expected as a promising driving power source. For example, there is a so-called rocking chair type lithium ion rechargeable battery comprising a negative electrode using as an active material a carbon material capable of reversibly intercalating and deintercalating lithium ions, a positive electrode using a lithium-containing transition metal complex oxide as an active material, a separator and a non-aqueous electrolyte. This battery has already been placed into practical use and is rapidly spreading.
As the active material for the negative electrode, graphite particles having higher crystallinity among various carbon materials have recently been prevailed for the following reasons: (1) electronic conductivity is high and discharge performance under a large current becomes excellent; (2) variations in potential accompanying discharge become small, which is suitable for discharge under constant electric power; and (3) a true density is high and hence a bulk density becomes high, which is suitable for making the energy density of the battery higher.
The graphite materials which are now under development or consideration for the negative electrode of the lithium rechargeable battery are mainly as follows:
(I) massive natural graphite particles prepared by making flake-like particles spherical in a process of pulverizing crude ore;
(II) artificial graphite particles prepared by graphitizing a certain kind of coke or granules made of coke and various pitches; and
(III) special artificial graphite particles derived from mesophase carbon (a kind of liquid crystal) generated by heating pitch or tar, examples of which are (1) a material prepared by carbonizing and graphitizing an extract of mesophase microspheres (graphitized mesocarbon microbeads: abbreviated as graphitized MCMB), (2) a material prepared by spinning fused mesophase pitch formed by polymeric growth of the mesophase microspheres, oxidizing the surface to make it infusible, followed by carbonizing, cutting, pulverizing and graphitizing (milled graphite fibers or graphitized milled mesocarbon pitch-based fibers: abbreviated as graphitized milled-MCF) and (3) a material prepared by carbonizing and graphitizing pulverized bulk mesophase pitch which is less fusible and formed by polymeric growth of the mesophase microspheres (graphitized bulk mesophase pitch).
Various attempts have been made to give high performance to the above-described graphite materials in view of a recent demand for the high energy density lithium rechargeable battery. Since the natural graphite particles of item (I) are capable of showing reversible capacity close to a theoretical capacity of graphite (372 mAh/g), research has eagerly been carried out on adjustment of the particle form suitable for filling the particles in an electrode in high density. Further, as described in Japanese Laid-Open Patent Publication No. HEI 11-54123, an edge surface exposed on the graphite particle surface is covered with amorphous carbon, which is classified as easily graphitizing carbon, to reduce irreversible capacity accompanying electrolyte decomposition caused on the surface of the graphite particles at the initial charge. On the other hand, the artificial graphite particles of items (II) and (III) are currently not capable of showing reversible capacity close to the theoretical capacity of graphite (poor in reversible capacity as compared with the natural graphite). Therefore, attempts have aggressively been made to enhance the degree of graphitization of the particles and to improve the reversible capacity by increasing purity of raw materials such as coke, pitch and tar, optimizing the graphitizing conditions in accordance with the materials, or adding a catalyst for promoting the graphitization. In such artificial graphite materials, the edge surface exposed on the particle surface is small in area, and hence in many cases the irreversible capacity at the initial charge is generally smaller than that of the natural graphite of item (I).
In the actual production of the negative electrode of the lithium rechargeable battery, one or a mixture of two or more of the above-described graphite materials may be used as the active material. In general, to the graphite particles used as the active material, SBR (styrene-butadiene copolymer rubber) or the like as a binder, CMC (carboxymethyl cellulose) or the like as a thickener and water are added in an appropriate amount to prepare an aqueous paste. Or alternatively, PVDF (polyvinylidene fluoride) or the like as the binder/the thickener and NMP (N-methyl-2-pyrrolidone) or the like as a dispersion medium are added in an appropriate amount to the graphite particles to prepare an organic paste. This paste is applied to a copper core material, which is dried and rolled to have a desired thickness and density. The resulting product is then subjected to cutting and a lead is welded to the core material, thereby obtaining a negative electrode plate. In many cases, the upper limit of the density of the negative electrode material mixture layer is set at about 1.6 g/cm3 in view of the degree of crush or collapse of the negative electrode active material particles while rolling the layer in high density, as well as the degree of falling or peeling of the particles off the core material. Even so, a lithium rechargeable battery having an energy density by volume as high as over 350 Wh/L can be obtained by combining the negative electrode with a positive electrode mainly containing LiCoO2 as the active material and being rolled in high density and a separator made of a thin porous film of polyolefin having appropriate mechanical strength and porosity.
Since portable equipment can easily be designed to be compact and thin in recent years, there are growing needs for a high energy density lithium rechargeable battery with added values of “thin and lightweight”, in which a negative electrode, a positive electrode and a separator are generally wound in the form of an almost rectangular column or an elliptic cylinder to form an electrode group, the electrode group is sealed in a prismatic metal case or a case made of an aluminum foil/resin film laminate and a non-aqueous electrolyte is poured therein.
There are various capabilities that are required for the above-described lithium rechargeable battery. In view of obtaining much higher energy density, generally adopted is a method of rolling the negative electrode material mixture layer formed on the copper core material in higher density (more specifically, about 1.6 to 1.8 g/cm3 as the density of the coated negative electrode material mixture layer including the binder and the like). However, since the true density of graphite is 2.22 to 2.24 g/cm3, the density of the mixture layer exceeding 1.6 g/cm3 represents a state in which the mixture layer is rolled in extremely high density. Therefore, in the step of rolling the coated negative electrode material mixture layer with a roller or the like, problems are apt to occur, e.g., the mixture layer cannot be rolled down to a predetermined thickness, and the falling or peeling of the mixture layer off the core material becomes manifest.
Such problems often result from the kind of graphite particles used as the negative electrode active material. In light of the inventors' empirical rule, the special artificial graphite particles of item (III) derived from mesophase carbon are liable to cause the former problem, i.e., the negative electrode material mixture layer cannot be rolled in high density. Further, the artificial graphite particles of item (II) derived from coke and the like are apt to cause the latter problem of falling or peeling of the mixture layer off the core material. As to the special artificial graphite particles of item (III) derived from mesophase carbon, poor slippage between the particles is considered as a cause of the problem. For the purpose of alleviating fusion of the particles in the steps of carbonization and graphitization, it is necessary to give the mesophase particles treatment for making their surface layers infusible (mild oxidation treatment). The resulting surface layers of the particles are in an amorphous state where the graphitization is not proceeding so much. That is, the state of contact among the artificial graphite particles (III) as the active material in the negative electrode material mixture layer is substantially that of amorphous carbon. The amorphous carbon having no layer structure causes less electrostatic repulsion between particles (interaction between π electrons), which is a phenomenon unique to the layer structure of graphite, and hence is poor in slippage. Therefore, if such materials are used as the negative electrode active material, the problem is apt to occur, i.e., the resulting negative electrode material mixture layer cannot be rolled in high density. As a solution to the problem, for example, Japanese Laid-Open Patent Publication No. 2001-236950 proposes a technique for forming the negative electrode material mixture layer by adding massive natural graphite particles or flake-like natural graphite particles as an auxiliary material to the graphitized MCMB.
The artificial graphite particles of item (II) derived from coke and the like are generally formed by graphitization and subsequent pulverization for adjusting the particle size. In many cases, it is difficult to obtain particles having a high bulk density (or a tap density) or a small specific surface area. This is considered to be a cause of the peeling and falling of the mixture layer while rolling the mixture layer in high density. That is, the particles are crushed or collapsed through the rolling of the mixture layer in high density because of the bulkiness of the particles. Further, most of the binder added to the mixture layer is adsorbed onto the particle surfaces because of the high specific surface area of the particles. Therefore, the binding state between the core material and the particles or between the particle and particle cannot be well maintained. For this reason, it is inferred that the falling and peeling of the mixture layer is apt to occur while rolling the layer in high density.
As compared with the above artificial graphite particles, the natural graphite particles of item (I) are basically graphitized enough to the surface layer, and hence show strong electrostatic repulsion between particles and extremely high slippage. Therefore, the rolling of the mixture layer in as high density as over 1.6 g/cm3 is relatively easy and problems accompanying the production are hard to arise. However, as described in Japanese Laid-Open Patent Publication No. HEI 11-263612, it is very difficult to shape the whole particles into almost complete spheres even if treatment is given to convert the flake-like particles into massive (spherical) ones. As a matter of fact, a large number of spindle-shaped (flat) particles having considerably large aspect ratio are included. Accordingly, in the case where the rolling is performed to such a degree that the density of the mixture layer exceeds 1.6 g/cm3, there occurs a phenomenon in which the spindle-shaped particles are oriented in a plane direction of the core material while involving partial deformation of the particles (a phenomenon which is well known with the conventional flake-like natural graphite particles), though it depends on the degree of the control of the particle shape. As a result, characteristic problems may occur, for example: (1) the edge surface of the graphite particle through which Li ions are intercalated and deintercalated is hard to expose to the electrolyte, which decreases the diffusibility of the Li ions and deteriorates a high rate discharge characteristic; and (2) expansion and contraction in the c-axis direction of the graphite particles during charge and discharge are apt to be reflected as the change in thickness of the mixture layer (the degree of expansion and contraction of the electrode is great).
Thus, in the step of rolling the mixture layer in high density, the natural graphite particles are oriented (i.e., graphite crystal forming the particles is oriented) and the electrode performance is deteriorated. Taking this problem into consideration, Japanese Laid-Open Patent Publications Nos. 2001-89118 and 2002-50346 propose a technique of mixing a graphitizable raw material (mostly coke and the like) with a graphitizable binder (tar or pitch), carbonizing the mixture, followed by pulverization and graphitization to prepare artificial graphite in which graphite structure or crystal is oriented at random, and applying the resulting artificial graphite to a negative electrode of a lithium rechargeable battery. Similar to this technique, for example, Japanese Laid-Open Patent Publication No. 2001-357849 describes that kish graphite (recrystallized graphite) obtained in a steel-making process is granulated with a binder, which is graphitized and used in the negative electrode. With the thus obtained artificial graphite particles, the problem of falling and peeling of the mixture layer is apt to occur. However, the graphite crystal existing at random in the particles is not affected even if the particles are oriented in the plane direction of the core material during rolling the electrode in high density. Therefore, it is relatively easy to avoid the above-described problems (1) and (2).
In a recent lithium ion rechargeable battery designed to have as high energy density as over 350 Wh/L, the negative and positive electrode active materials need to be filled in a larger amount in a battery case of a predetermined volume. Accordingly, remaining space in the battery (internal volume of the battery case from which volumes of the positive and negative electrodes, the separator and the like are subtracted) is reduced, which extremely decreases the ratio (cc/mAh) of the electrolyte amount to the designed battery capacity. As a result, the electrolyte does not permeate sufficiently into the inside of the negative electrode mixture layer rolled in high density, which deteriorates a high rate charge/discharge characteristic and a discharge characteristic at low temperature. This problem has not been caused in a conventional battery containing the electrolyte in a relatively large amount. A solution to this problem, which is partially disclosed by Japanese Laid-Open Patent Publication No. 2000-90930, is the use of, as the negative electrode active material, graphite particles being capable of maintaining an appropriate mean circularity (sphericity) even after the rolling (pressure molding), having a certain average particle diameter (10 to 35 μm), containing a less amount of fine powder which has a diameter of 4 μm or less and showing relatively sharp particle size distribution. Therefore, the natural graphite particles made into massive (spherical) as described above or the artificial graphite particles in which the graphite crystal is oriented at random may be used with less deterioration in high rate charge/discharge characteristic and discharge characteristic at low temperature if their particle sizes are optimally adjusted.
Even so, the lithium rechargeable battery designed to have high energy density has another problem, i.e., the reduction in capacity through the charge/discharge cycles is caused greater than in the conventional battery. Causes thereof are considered as follows. If the graphite particles used as the negative electrode active material are crushed or collapsed through the charge/discharge cycles, an edge surface of the crushed particles is exposed to the electrolyte to decompose and consume the electrolyte whose absolute amount is small from the start. As a result, the internal resistance of the battery increases. Further, a product generated through the electrolyte decomposition is deposited as a membrane on the negative electrode surface, which decreases the charge/discharge efficiency of the negative electrode.
In addition, regarding the high energy density lithium rechargeable battery of the latest-type formed by winding a positive electrode, a negative electrode and a separator into an electrode group in the form of an almost rectangular column or a elliptic cylinder, sealing the electrode group in a prismatic metal case or an aluminum foil/resin film laminate case and pouring therein a non-aqueous electrolyte, the battery case is generally low in strength. Accordingly, once the decomposition of the electrolyte occurs through the repeated charge/discharge cycles, cracked gas is generated to raise the internal pressure of the battery, thereby the battery is deformed (swelled) in a thickness direction. Further, the electrode group wound in the form of an almost rectangular column or an elliptic cylinder is more liable to be deformed through the expansion and contraction of the negative electrode material mixture than an electrode group wound into the form of a cylinder (wound in a spiral fashion) used in a cylindrical battery. These factors conspire to drastically reduce a cycle life characteristic of the battery.
It is possible to assume that the cycle life characteristic of the lithium ion rechargeable battery designed to have a high energy density is deteriorated in accordance with the above-described mechanism. Thus, a possible measure for improvements is to use, as the negative electrode active material, (1) graphite particles which are hard to crush or collapse through the charge/discharge cycles (poor in reactivity with the electrolyte during the charge/discharge cycles) with a view to inhibiting the decomposition and consumption of the electrolyte and (2) graphite particles which do not expand or contract to a great extent during the charge/discharge cycles. As a result of intensive study on various graphite materials performed by the inventors of the present invention, it was found that the massive natural graphite particles (or those subjected to surface reforming or surface coating) used as the negative electrode active material were crushed or collapsed through the charge/discharge cycles to a greater extent than the artificial graphite particles. Even in a system where the electrolyte was added with a currently known additive for forming a protective membrane on the negative electrode at the initial charge to inhibit the electrolyte decomposition through the charge/discharge cycles (a typical example of the additive is vinylene carbonate), a satisfactory cycle life characteristic was not obtained. It was also found that the above-described artificial graphite particles containing the graphite crystal oriented at random were favorable because the degree of crush or collapse of the particles through the charge/discharge cycles was small, as well as the degree of the expansion and contraction through the charge/discharge cycles was relatively small.
On the other hand, the artificial graphite particles made by the technique disclosed in Japanese Laid-Open Patent Publications Nos. 2001-89118 and 2002-50346 are firmly fused with each other in the steps of carbonization and graphitization during the manufacture thereof. Therefore, intense pulverization is required after the graphitization. As a result, the obtained graphite particles become large in specific surface area. The specific surface area of the graphite particles in the negative electrode is empirically known to have a correlation with the initial irreversible capacity and the thermal stability (heat resistance under the charged state) of the negative electrode. If the particles have a large specific surface area, the initial irreversible capacity increases and the thermal stability decreases. Therefore, it is unfavorable in view of giving high capacity and stability to the battery.
Considering the above, Japanese Laid-Open Patent Publication No. HEI 11-199213 discloses a method of manufacturing artificial graphite by mixing a graphitizable raw material (coke) and a graphitizable binder (tar or pitch), carbonizing the mixture, slightly pulverizing the carbonized mixture and graphitizing the resulting powder (i.e., the pulverization is carried out before the graphitization). The publication also discloses that the specific surface area of the particles is reduced to 1.0 to 3.0 m2/g by: (1) making the mixture infusible before the carbonization by oxidizing the binder; (2) adding thermosetting resin to the binder to prevent fusion of the particles during the carbonization; or (3) coating the mixture of the raw material and the binder with thermosetting resin. Further, according to the embodiments, is described an example of preparation of artificial graphite particles having an average particle diameter (D50) of 25 to 30 μm and a specific surface area of 1.8 to 2.2 m2/g measured by a BET method.
However, as long as specific starting materials (coke, tar and pitch in the case of the above-described publication) are used, there is a limitation in reducing the specific surface area of the particles. For example, if D50 (25 to 30 μm) of the graphite particles according to the embodiment of the above-described publication is slightly reduced to about 20 μm to alleviate the settlement of the negative electrode material mixture paste (to make the paste easy to handle during the manufacture so as to increase yield), the BET specific surface area exceeds 3 m2/g. Thereby, the negative electrode increases in initial irreversible capacity and deteriorates in thermal stability (heat resistance). Further, the graphite particles of the above-described publication are smaller in bulk density (or tap density) than the other graphite particles. That is, the graphite particles have another disadvantage that the negative electrode material mixture layer easily falls off the core material while being rolled into a high density electrode, and hence are susceptible to improvement also in this point.